Hello Mm_Litsardaki,
Thanks for asking the Apple Support Communities about buying a gift card. We're happy to offer some info.
To buy a gift card, you need a payment method from the same location as your Apple ID: Payment methods that you can use with your Apple ID - Apple Support
Even if you could, gift cards can only be redeemed in the country they were purchased from: What type of gift card do I have? - Apple Support
Notice this article: If you can't redeem your App Store & iTunes Gift Card, Apple Music Gift Card, or content code - Apple Support
If you see a message that says the code must be redeemed in another storefront, the card was probably purchased in another country or region. You can't redeem Apple Gift Cards or App Store & iTunes Gift Cards outside of the country or region of purchase. For example, if you bought an App Store & iTunes Gift Card in France, you can't redeem it in the App Store in the United States. If you need more help, contact Apple Support."
Hope this helps. Thanks.